There is another thread here in this forum "P650 driver on Ubuntu 8.10 (2.6.27-7) fails to compile?" with a discussion on how to change some parts of the system to be able to use the matrox mtx driver with Ubuntu 8.10.

Is it still necessary to do all this things (that are not very clear to me)? Or has something changed with the driver or Ubuntu or whatever?

Correct and unfortunately, that's your only choice at the moment. Ubuntu can't do anything about that, because they're moving forwards and Matrox doesn't so the latest available mtx drivers does not work with the latest available Xorg version anymore.

You can downgrade the installed Xorg on Ubuntu 8.10 which works a treat and lets you install the driver there as outlined in the other post you referred to.